The presence of al Qaida-linked groups fighting alongside rebels in Syria continues to grow – and reshape the conflict.

With that expanding role comes mounting unease among some – including much of the mainstream of the Syrian rebellion against President Bashar Assad – about manning the same side of the ramparts as forces known primarily for terrorism.

The Islamic State of Iraq and the Levant, as the local al Qaida affiliate is known, has fast been expanding its influence across virtually all rebel-held areas in northern Syria. It’s fighting alongside its ideological ally, the Nusra Front.

And it’s brought to the fight brutal tactics that marked the insurgency against U.S.-led forces in Iraq. That’s left aid workers, Western patrons of the revolution and others jumpy about such an alliance. The jihadists’ role has at times also pitted rebel against rebel in the already chaotic and shifting battlefield in the Syrian civil war.

Both the al Qaida and Nusra groups are led by veterans of the Iraqi insurgency, and both have flirted with the tactics that ultimately alienated them among Sunnis in that country who ultimately turned against them.

Still, mainstream rebel factions have been reluctant to denounce the fierce Islamist militants they now find siding with them against the Damascus regime. That may be partly a factor of success.

When rebels early this month made the critical seizure of an airbase in Idlib province, they dealt the Syrian regime a serious blow and opened up critical supply lines for their cause. They also scored the bloody victory with the help of the al Qaida group.

Likewise, those Islamist and mostly foreign forces spearheaded a high-profile offensive into the coastal region – territory previously seen as unassailable because the Alawite Muslim population supports the Assad government.

In both cases, the mainstream rebels’ Free Syrian Army and Syrian Military Council publicly thanked the Islamist fighters in a series of videos. Those announcements were released by umbrella groups, who face stiff pressure from the West to confront their more radical allies.

The uptick in military successes by the Islamists comes as more mainstream factions continue to suffer major setbacks – particularly previously rebel-held towns along the Lebanese border. Both Homs and Qusayr, previously territory belonging to the rebel Farouk Brigade, fell to the regime this summer.

Few would dispute that the revolution remains under the control of the Syrian Military Council. But the military wins lately have come with distinctive fingerprints of more radical Islamists.

“While other battalions might like to claim credit, the fact is that ISIS” – the acronym attached to the al Qaida group – “banner is flying over the main tower at the airbase,” said Aymenn Tamimi, an analyst specializing in Iraqi and Syrian jihadist groups at Oxford University. “(The Islamists are) more successful in terms of control of territory and influence than their counterparts in Iraq could ever hope to have achieved.”

That influence took on a fractious tone last week. The al Qaida fighters grew frustrated that their ostensible rebel allies were too timid in efforts to seize the provincial capital of Raqqa. The al Qaida group dispatched a series of car bombs – not to attack regime forces, but against rebel headquarters. That was followed by ground assaults and the execution of 18 rebel fighters by the Islamists, according to the London-based Syrian Observatory for Human Rights.

“They killed our commanders and took control of Raqqa, which they say will be the seat of a new Islamic state in Syria,” said one rebel commander who had fled Raqqa for the rebel-held city of Deir Azour. “These maniacs are more dangerous than the regime to me at this point.”

That commander refused to be identified even by a pseudonym because “they already want to kill me and they read all the articles about themselves in the Western press.”

And yet the incident in Raqqa marked just the latest violent flashpoint between the disparate forces that sometimes come together, and sometimes don’t, in opposition to the Assad regime.

Hundreds of fighters from Europe’s large Muslim communities have joined the jihadists. They often divide into brigades built around common languages – Russian speakers from the Caucuses, British of Pakistani descent, and so on.

The foreign jihadists tend to pass mostly through Turkey, which shares more than 300 miles of border with Syria. That gives the foreign fighters ready access to the northernmost battlefields. In places to the south such as Deraa, Damascus and Homs province, the non-Syrian Islamists pose far less influence.

Both the Nusra Front and its al Qaida-linked allies have battled viciously with Syrian Kurds – the country’s largest ethnic minority – for control over the far-eastern portion of the country. That’s marked the worst intramural fighting among the rebels.

Nusra has proven the more adept of the two at winning local support. Veterans of the Iraqi insurgency see that as critical. Indeed, the tactics that hit military and civilian targets alike ultimately turned many communities, through their “Awakening Councils,” against the al Qaida groups in Iraq and made them allies of convenience with the American-led occupying forces.

Nusra has generally avoided conducting brazenly sectarian attacks and harassment of local residents in Syria. Instead, it has promoted security services and bread giveaways.

In contrast, the al Qaida affiliate has harassed civilians and waged a campaign of kidnappings of Westerners – including journalists and, most notably, an Italian priest. The Rev. Paolo Dall’Oglio went missing earlier this month after he tried to negotiate a truce between the jihadists and Kurdish factions.

“(The Islamist group) is trying to establish itself as the de facto ruler of those areas of Syria where they have the most influence,” said Eliot Higgins, who closely monitors developments in Syria for a prominent blog, brown-moses.blogspot.com. “They are also preparing themselves for a conflict with other oppositions groups.”

He sees the rebellion as just the first stage in what figures to be a prolonged conflict between Islamists and more secular forces.